An image capture device depends on an electronic image sensor to create an electronic representation of a visual image. Examples of such electronic image sensors include charge coupled device (CCD) image sensors and active pixel sensor (APS) devices (APS devices are often referred to as CMOS sensors because of the ability to fabricate them in a Complementary Metal Oxide Semiconductor process). Typically, electronic image sensors are used for the multiple functions related to pre-photography preparations from the video signal in addition to creating the final visual image. Based on brightness measurement results of the subject, automatic exposure control processing (hereinafter referred to as “AE processing”) is carried out to obtain a suitable exposure value. Then, automatic focus detection processing (hereinafter referred to as “AF processing”) is carried out to drive a focus-adjusting lens to focus the subject on the image capture device. The subject brightness value is measured from the video signal again, and photographic exposure conditions are thereby determined. In addition to AE, AF, and other analytical processing, image capture devices often display a visual electronic image of the scene to be captured. This visual image is updated frequently, such as 30 frames per second, and is referred to as a preview image or stream of preview images.
Commonly, a single electronic image sensor is used for creating the electronic representation of a visual image, AE processing and AF processing. These tasks are performed sequentially since the same electronic image sensor is being utilized for different functions. Typically, the rate at which the AE processing and AF processing can be performed is restricted by the rate at which a visual image can be read and processed from the electronic image sensor. This can cause a considerable delay in time between when the electronic image sensor initiates processing and when the final capture is finally acquired.
In prior art, the user adjusts the zoom setting and points the camera to compose the image, and then actuates a capture device through user inputs. The camera focus is adjusted to a mid-range position, and the sensor is cleared of any charge. For example, with a CCD sensor, this would be done using a fast flush technique. An image, to be used for focusing the camera lens, is then integrated for a period of time, for example 10 milli-seconds, during the focusing mode. The vertical clock sequence is then set to a line skipping operation (e.g., read two lines, dump six lines, read two, dump six, etc.), or read only selected lines in the central area of the image.
After data acquisition, the average absolute value output (average contrast) of a horizontal spatial bandpass filter processing these image lines is used to determine how well the image is focused. The system controller stores this average contrast value, and the lens focus is adjusted while the remainder of the sensor charge is cleared out using fast flush timing. The fast flush timing for the top and bottom of the sensor are required with a CCD in order to reduce the time spent reading out each focus image. Sensor lines that are flushed are not available for any purpose, such as exposure analysis or video signal output. The process of integrating and reading out the focus image is then repeated for a second focusing cycle. If the average contrast increases, the lens focus position is stepped again in the same direction. If the average contrast decreases, the focus position is moved in the opposite direction. These focusing cycles are repeated numerous times as the lens focus is adjusted until it provides the maximum average contrast. Once the average contrast has reached a maximum value, the focus is acceptable. At this point, the entire sensor is cleared. The final image is then integrated for a period of time. The final image is read out from the sensor.
Prior art also includes focus analysis techniques besides the average contrast calculation described above. Still, they rely on a through focus operation, acquiring multiple images at different focus positions.
In order to solve the time problem, some capture devices actually have two image sensors: one that operates at a fast frame rate to provide for AE or AF processing and the other that operates at a slow frame rate for producing a visual image signal. This of course involves the complexity of a second sensor and its control. The added complexity includes optical and mechanical complexity as well as electronic complexity.